eclipse + JBoss 5 + EJB3开发指南（9）

eclipse + JBoss 5 + EJB3开发指南（9）2011-09-18 blogjava 哈佛校训实现Entity Bean的多对多（many-to-many）映射

在EJB3中需要使用@ManyToMany对封装多对多关系的字段或getter方法进行注释。先看看下面的表：

图1 t_addresses表

t_addresses表和t_customers表是多对多的关系。需要使用一个关联表来描述这种关系，关联表的结构如下图所示。

图2 t_customers_addresses

在Customer类中定义一个Collection<Address>类型的字段（addresses），用于保存与该 Customer对象相对应的多个Address对象，代码如下：

package entity;import java.util.Collection;import javax.persistence.CascadeType;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.JoinTable;import javax.persistence.ManyToMany;import javax.persistence.OneToMany;import javax.persistence.OneToOne;import javax.persistence.PrimaryKeyJoinColumn;import javax.persistence.Table;@Entity@Table（name = "t_customers"）public class Customer{    private int id;    private String name;    private Referee referee;    private Collection<Order> orders;    private Collection<Address> addresses;    @ManyToMany（cascade = CascadeType.PERSIST, fetch = FetchType.LAZY）    @JoinTable（name = "t_customers_addresses", joinColumns = @JoinColumn（name = "customer_id",      referencedColumnName = "id"）, inverseJoinColumns = @JoinColumn（name = "address_id", referencedColumnName = "id"））    public Collection<Address> getAddresses（）    {        return addresses;    }    ... ...}